The invention relates to positioning systems, such as the well known global positioning system, and more particularly to power consumption in receivers of signals transmitted by reference elements of such positioning systems, such as the satellites of the global positioning system.
In satellite positioning systems (such as the global positioning system or GPS), the position of a receiver (user) and its time offset from the system time (i.e. the correction to the receiver time at which the receiver is determined to be at the determined position) can be determined by using (pseudorange) measurements obtained from information (ephemerides and C/A-code phases) provided from at least four satellites. Such a determination can use satellite measurements at a particular instant of time, in what is called a single-point solution, a solution that in no way takes into account past information obtained from the satellites; any error in the measurements obtained from the satellites, including error from noise or multi-path, is reflected in such a single-point solution.
Filtering with a Kalman filter (or some modification of such a filter) can instead be used to enhance the quality of the receiver""s estimated track (by providing smoother, less noisy solutions than are provided by a single-point solution), and also to provide useable solutions in periods when satellite measurements are not available (because of poor signal conditions). The performance of any such filter is dependent on how the receiver""s motion is modeled in the filter.
Usually, instead of using an ordinary Kalman filter, what is called an extended Kalman filter (EKF), which is a linearized form of Kalman filter, is used, because a standard Kalman filter assumes that the measurement update equations are linear, and for positioning problems the measurement update equations, which involve the pseudoranges, are nonlinear. For a (standard) Kalman filter to be used, there has to be a linear relationship between the measurement vector m and state vector s, such that m=Hxc2x7s, where H is some matrix. In GPS positioning, if the state vector is for example of the form [x y z t], where (x,y,z) indicates position and t represents clock bias, there is no such linear equation between pseudorange measurements and state. Instead, the ith component of the measurement vector (i.e. the pseudorange from the ith satellite), is given by
m(i)={square root over ((xixe2x88x92x)2+(yixe2x88x92y)2+(zixe2x88x92z)2)}+xcex94t,
(where xcex94t is a clock error/bias term) which is obviously not a linear relationship. In an EKF, to be able to still use a Kalman type filter in an application where such a nonlinear relationship exists, the nonlinear relationship is approximated by a linear relationship by forming a truncated Taylor series of the nonlinear equation and taking the first, linear term of the series. In practice, this means that the H matrix in the equation m=Hxc2x7s is approximated by the so-called Jacobian (known in the art) of the pseudorange equations.
Thus, in an EKF, a standard Kalman filter (for linear systems) is applied to nonlinear systems (with additive white noise) by continually updating a linearization around a previous state estimate, starting with an initial guess. In other words, a linear Taylor series approximation (no nonlinear terms) of the system function at the previous state estimate is made, and a linear Taylor series approximation of the observation function at the corresponding predicted position. Such an approach yields a relatively simple and efficient algorithm for handling a nonlinear model, but convergence to a reasonable estimate depends to a great extent on the accuracy of the initial guess at the desired position; the algorithm may not converge if the initial guess is poor or if disturbances to the motion are so large that linearization is inadequate to describe the system.
The prior art also teaches using what is called the interacting multiple model (IMM) solution, in which various multiple models are assumed for the motion of the receiver (modules assuming slow turning, fast turning, slow accelerating, fast accelerating, and so on), and the outputs of the different models are combined based on weights that take into account how the predictions of the model agree with later measurements made on the basis of later information received from the satellites.
In such an approach, each model (branch of the IMM solution) is implemented as an EKF.
GPS satellites broadcast navigation data (including their ephemerides and health information) using a direct sequence spread spectrum signal. Doing so allows all of the satellites to share the same frequency spectrum. Each satellite modulates the same carrier frequency with a pseudorandom number (PRN) code (via binary phase shift key modulation) as well as with the navigation data for the satellite. A GPS receiver must acquire and track the signal from a GPS satellite in order to read the navigation data from the satellite. The acquisition and tracking of a GPS signal for a particular one of the GPS satellites amounts to synchronizing the received PRN code for the GPS satellite (obtained from the received signal after removing the carrier frequency) with a replica of the PRN code generated by the GPS receiver. A correlator determines at what relative position the replica PRN code is in phase with the received PRN code.
When a GPS receiver is integrated into a mobile phone, power consumption by the mobile phone is significantly increased. Since only a limited reservoir of power is available, every module inside a mobile phone should consume as little power as possible. Besides a GPS module and the required cellular transceiver module, examples of modules that could be included in a mobile phone are a so-called BlueTooth receiver module, a WLAN (wireless local area network) module, and a camera module.
There are many ways to reduce power-consumption of a mobile phone having component modules, such as miniaturizing the component modules, selecting more power-efficient component modules, and shutting down (temporarily) a component module partially (i.e. shutting down some parts of the component module) or completely (shutting down the component module entirely). Of course a partial or complete shutdown of a component module is possible only if an acceptable level of service can still be provided by the affected component module.
What is needed is a way to reduce power consumption by a GPS receiver without significantly degrading performance of the GPS receiver positioning function. Such a way to reduce power consumption would be especially advantageous in case of a GPS receiver operating as a component of a mobile phone.
Accordingly, a first aspect of the invention provides a method for conserving power in a positioning system receiver used in connection with a positioning system providing ranging signals, the receiver using the ranging signals to determine a state of motion of the receiver, the method including: a step of performing at least a predetermined number of solutions of the state of motion of the receiver using a filter solution based on a mix of models of the motion of the receiver, a mix that is varied from one solution to the next according to a predetermined criteria, and of providing the model mix used in each solution; and a step of adopting a partial duty cycle indicating a percentage of time selected receiver components are powered on, based on the mix of models used in successive solutions.
In accord with the first aspect of the invention, the receiver may include a radiofrequency (RF) front end module and a baseband processor module and the selected components may include the RF front end module. Further, the selected components may also include the baseband processor module.
In a second aspect of the invention, an apparatus is provided for conserving power in a positioning system receiver used in connection with a positioning system issuing ranging signals, the receiver using the ranging signals to determine a state of motion of the receiver, the apparatus including: means for performing at least a predetermined number of solutions of the state of motion of the receiver using a filter solution based on a mix of models of the motion of the receiver that are varied from one solution to the next according to a predetermined criteria, and for providing the model mix used in each solution; and means for determining a partial duty cycle indicating a percentage of time selected receiver components are powered on, based on the mix of models used in successive solutions.
In accord with the second aspect of the invention, the receiver may include a radiofrequency (RF) front end module and a baseband processor module and the selected components may include the RF front end module. Further, the selected components may also include the baseband processor module.
In a third aspect of the invention, a system is provided, including: a transmitter for transmitting a ranging signal, and a ranging receiver for receiving the ranging signal and for determining a state of motion of the ranging receiver, the ranging receiver characterized in that it includes an apparatus for conserving power that in turn includes: means for performing at least a predetermined number of solutions of the state of motion of the ranging receiver using a filter solution based on a mix of models of the motion of the ranging receiver that are varied from one solution to the next according to a predetermined criteria, and for providing the model mix used in each solution; and means for determining a partial duty cycle indicating a percentage of time selected ranging receiver components are powered on, based on the mix of models used in successive solutions.
In accord with the third aspect of the invention, the system may also include a computing resource external to the ranging receiver, and the apparatus may communicate information to the computing facility via a wireless communication system and the computing facility may use the information in assisting the apparatus in performing at least a predetermined number of solutions of the state of motion of the ranging receiver using a filter solution based on a mix of models of the motion of the ranging receiver that are varied from one solution to the next according to a predetermined criteria.
By allowing some component of a ranging receiver (i.e. the receiver front end and baseband processing module) to be turned on and off during operation of the ranging receiver, the invention allows substantial savings in power compared to what is provided by the prior art, which does not teach turning on and off components of a ranging receiver. The savings can amount to a reduction in power used by the components turned on and off for example in some embodiments by as much as a factor of from two to fifty. For example, a ranging receiver using the invention can have its receiver front end and baseband processing module powered on only 300 ms out of every other second, for a reduction in power by a factor of six (2000 ms/300 ms), or the selected receiver components could be turned on only every 100 ms every five seconds, for a reduction in power of a factor of 50 (5000 ms/100 ms).